To meet the demands of the high performance of electronic systems such as personal computers or electronic communication devices, nonvolatile semiconductor memory devices such as DRAMs mounted as memory devices have been designed to operate at higher speed and have become highly integrated. Since semiconductor memory devices mounted on battery-driven systems such as mobile phones or notebook computers require a low power consumption characteristic, many efforts and developments have been made to reduce an operating current and a standby current.
A data retention characteristic of a DRAM cell consisting of one transistor and one storage capacitor is sensitive to temperature. Therefore, it may be necessary to adjust the operating conditions of circuit blocks inside semiconductor integrated circuits, depending on the variation in an ambient temperature. For example, DRAMs used in mobile products adjust a refresh period, depending on the variation in an ambient temperature. Temperature sensors, such as a Digital Temperature Sensor Regulator (DTSR), an Analog Temp Sensor Regulator (ATSR), and a Digital Temperature Compensated Self Refresh (DTCSR), are used to adjust the operating conditions depending on the variation in the ambient temperature.
FIG. 1 is a block diagram illustrating the configuration of a conventional temperature sensor. The conventional temperature sensor includes a sense voltage generation unit 10 and a temperature code generation unit 11. The sense voltage generation unit 10 is configured to sense an inside temperature of a semiconductor integrated circuit and to generate a sense voltage VSENSE based thereon. The temperature code generation unit 11 is configured to compare the sense voltage VSENSE with a reference voltage VREF and to generate a temperature code TQ based thereon. More specifically, the temperature sensor compares a level of the sense voltage VSENSE with a level of the reference voltage VREF and generates the temperature code TQ, which contains information indicating whether an inside temperature of the semiconductor integrated circuit is higher than a temperature corresponding to the level of the reference voltage VREF.
However, such a temperature sensor can sense only one temperature, therefore it is necessary to adjust the level of the reference voltage VREF through a design modification in order to sense multiple temperatures. Furthermore, when the level of the sense voltage VSENSE changes according to process variations, a design modification for adjusting the level of the reference voltage VREF is required.